Transmission of video signals over significant distances is limited by broadcast transmission range. Depending on the communication method, video signals experience different types of degradation. Some video signals, such as NTSC (National Television System Committee) signals, experience graceful degradation so that viewers on the "fringe" of a viewing area receive a "fuzzy" but viewable picture. Particularly in all-digital transmission systems, signal degradation occurs at the fringe of the viewing area due to the sensitivity of digital transmission to small variations in the signal-to-noise ratio. When the signal-to-noise ratio falls below a particular threshold, there can be significant signal degradation resulting in loss of picture for up to several seconds, which is generally not considered acceptable by the broadcast industry.
Attempts have been made to increase the viewing area of video transmissions by decreasing sensitivity to noise. A number of commonly assigned United States patents disclose various techniques that provide graceful degradation in the reception quality at a TV set location for digital television signals. These are V.B. Lawrence et al. entitled "Coding for Digital Transmission," U.S. Pat. No. 5,164,963 (Nov. 17, 1992); L.-F. Wei entitled "Coded Modulation with Unequal Error Protection." U.S. Pat. No. 5,105,442 (Apr. 14, 1992); and Johnston et at. entitled "A High Definition Television Coding,; Arrangement with Graceful Degradation", U.S. Pat. No. 5,128,756. In addition, u co-pending commonly assigned U.S. Patent application of H.Y. Chung et al. entitled "Multiplexed Coded Modulation with Unequal Error Protection," Ser. No. 07/627.156, filed on Dec. 13, 1990 discloses a technique for providing graceful degradation in reception quality. However, the techniques of these patents and the patent application attempt to protect partial picture quality over the entire screen. In these schemes data protection is applied to the more important components of the compressed data.
One approach to decreasing the effects of signal degradation at the fringe of the viewing area is to transmit the more important data at a different channel rate from the less important data. The me, re important data are transmitted at a channel rate having less susceptibility to noise or other degradation. The resulting combination of video transmissions allows a more graceful degradation at the video receiver since the less important data will be affected first. Such an approach is described in co-pending, commonly assigned United States patent application of A.F. Faryar et. al. entitled "Soft Coding for HDTV", Ser. No. 07/785,671 filed on Oct. 31, 1991, now U.S. Pat. No. 5,243,419. While the general concept of classifying the data blocks of a video signal into different categories as described in tile Faryar reference is an advantageous technique, it is still necessary to select the categories to be transmitted at the various channel rates. One method described in Faryar is to classify the data blocks in accordance with the number of motion-vector bits.